This invention relates generally to nanowires and more particularly to nanowires having a diameter which is relatively small and uniform and techniques for fabrication thereof.
As is known in the art, a nanowire refers to a wire having a diameter typically in the range of about one nanometer (nm) to about 500 nm. Nanowires are typically fabricated from a metal or a semiconductor material. When wires fabricated from metal or semiconductor materials are provided in the nanometer size range, some of the electronic and optical properties of the metal or semiconductor materials are different than the same properties of the same materials in larger sizes. Thus, in the nanometer-size range of dimensions, the physical dimensions of the materials may have a critical effect on the electronic and optical properties of the material.
Quantum confinement refers to the restriction of the electronic wave function to smaller and smaller regions of space within a particle of material referred to as the resonance cavity. Semiconductor structures in the nanometer size range exhibiting the characteristics of quantum confinement are typically referred to as zero-dimension (OD) quantum dots or more simply quantum dots when the confinement is in three dimensions. Quantum dots are provided from crystalline particles having a diameter less than about ten nanometers which are embedded within or on the surface of an organic or inorganic matrix and which exhibit quantum confinement in three directions.
Similarly, when the confinement is in one dimension, the structures are referred to as 2D quantum well superlattices or more simply xe2x80x9cquantum wells.xe2x80x9d Such superlattices are typically generated by the epitaxial growth of multi-layer active crystals separated by barrier layers. The 2D quantum wells have typically enhanced carrier mobility and also have characteristics such as the quantum Hall effect and quantum confined Stark effect. 2D quantum well superlattice structures also typically have magnetoresistance and thermoelectric characteristics which are enhanced relative to 3D materials. One problem with quantum well superlattices, however is that they are relatively expensive and difficult to produce and fabrication of quantum well superlattices are limited to several material systems including group IV semiconductors (such as SiGe), group III-V compounds (such as GaAs), group II-VI compounds (such as CdTe) and group IV-VI compounds (such as PbTe).
When the quantum confinement is in two dimensions, the structures are typically referred to as a one-dimensional quantum wires or more simply as quantum wires. A quantum wire thus refers to a wire having a diameter sufficiently small to cause confinement of electron gas to directions normal to the wire. Such two-dimensional (2D) quantum confinement changes the wire""s electronic energy state. Thus, quantum wires have properties which are different from their three-dimensional (3D) bulk counterparts.
For example, metallic wires having a diameter of 100 nm or less have specific properties typically referred to as quantum conduction phenomena. Quantum conduction phenomena include but are not limited to: (a) survival of phase information of conduction electrons and the obviousness of the electron wave interference effect; (b) breaking of Ohm""s Law and the dependence of the electrical conductivity and thermal conductivity characteristics of the wire on the configuration, diameter and length of the metal; (c) greater fluctuation of wire conductivity; (d) noises observed within the material depend upon the configuration of the sample and the positions of impurity atoms; (e) a mark surface effect is produced; and (f) visible light enters throughout the thin wire causing a decrease in conductivity.
In transport-related applications, quantum wire systems exhibit a quantum confinement characteristic which are enhanced relative to quantum well systems. It is thus desirable to fabricate quantum wire systems or more generally nanowire systems for use in transport-related applications. One problem with nanowire systems, however, is that it is relatively difficult to fabricate nanowires having a relatively small, uniform diameter and a relatively long length.
One technique for fabricating quantum wires utilizes a micro lithographic process followed by metalorganic chemical vapor deposition (MOCVD). This technique may be used to generate a single quantum wire or a row of gallium arsenide (GaAs) quantum wires embedded within a bulk aluminum arsenide (AlAs) substrate. One problem with this technique, however, is that microlithographic processes and MOCVD have been limited to GaAs and related materials. It is relatively difficult to generate an array of relatively closely spaced nanowires using conventional microlithographic techniques due to limitations in the tolerances and sizes of patterns which can be formed on masks and the MOCVD processing required to deposit the material which forms the wire. Moreover, it is desirable in some applications, to fabricate two and three dimensional arrays of nanowires in which the spacing between nanowires is relatively small.
Another problem with the lithographic-MOCVD technique is that this technique does not allow the fabrication of quantum dots or quantum wires having diameters in the 1-100 nanometer range. Moreover, this technique does not result in a degree of size uniformity of the wires suitable for practical applications.
Another approach to fabricate nanowire systems which overcomes some of the problems of the lithographic technique, involves filling naturally occurring arrays of nanochannels or pores in a substrate with a material of interest. In this approach, the substrate is used as a template. One problem the porous substrate approach is that it is relatively difficult to generate relatively long continuous wires having relatively small diameters. This is partly because as the pore diameters become small, the pores tend to branch and merge partly because of problems associated with filling relatively long pores having relatively small diameters with a desired material.
Anodic alumina and mesoporous materials, for example, each are provided having arrays of pores. The pores can be filled with an appropriate metal in a liquid state. The metal solidifies resulting in metal rods filling the pores of the substrate. Surface layers of the substrate surrounding the rods are then removed, by etching for example, to expose the ends of the metal rods. In some applications the rods can be chemically reacted to form semiconductor materials. Some substrate materials, however, such as anodic alumina are not suitable host templates for nanowires due to the lack of a systematic technique to control pore packing density, pore diameter and pore length in the anodic alumina.
Nevertheless, porous materials such as anodic alumina have been used to synthesize a variety of metal and semiconductor nanoparticles and nanowires by utilizing chemical or electrochemical processes to fill pores in the anodic alumina. Such liquid phase approaches, however, have been limited to Nickel (Ni), Paladium (Pd), Cadmium sulfide (CdS) and possibly Gold (Au) and Platinum (Pt). One problem with chemical or electrochemical processes is that success of the processes depends upon finding appropriate chemical precursors. Another problem with this approach is that it is relatively difficult to continuously fill pores having a relatively small diameter and a relatively long length e.g. a length greater than 2.75 microns.
Still another approach to providing nanowires is to utilize an anodic alumina substrate to prepare carbon nanotubes inside the pores of the anodic alumina by the carbonization of propylene vapor. One problem with such a gas phase reaction approach is that it is relatively difficult to generate dense continuous nanowires.
To overcome the problems of filling pores in a template, high pressure-high temperature material injection techniques have been used. In these techniques, a molten metal is injected into relatively small diameter pores of a template to make nanostructure composites. In one technique, a hydrostatic press provides a relatively high pressure to inject metals such as indium (In), gallium(Ga) or mercury (Hg) into the pores of the substrate. This technique may also be used to fill a single glass nanotube having a diameter of about 100 nm with a molten metal such as bismuth (Bi). The technique may also be used to fill porous anodic alumina with channel diameters larger than 200 nm with various metal melts. One equation which may be used to compute the rate at which a molten metal can be injected into a pore of a template is shown in Equation 1:                               l          ⁡                      (            t            )                          =                                                            (                                                      P                    a                                    +                                      2                    ⁢                                          γ                      lv                                        ⁢                                          xe2x80x83                                        ⁢                    cos                    ⁢                                          xe2x80x83                                        ⁢                                          θ                      /                      r                                                                      )                            ⁢                              (                                                      r                    2                                    +                                      4                    ⁢                                          xe2x80x83                                        ⁢                                          ϵ                      ·                      r                                                                      )                            ⁢              t                                      4              ⁢                              xe2x80x83                            ⁢              η                                                          Equation        ⁢                  xe2x80x83                ⁢        1            
in which:
l(t) is an injection length at time t;
Pa is an external pressure;
xcex3lv is a liquid-vapor surface tension;
xcex8 is a contact angle between the liquid and a wall of the pore;
r is a pore radius;
xcex7 is a viscosity of the liquid; and
xcex5 is a coefficient of slip of the liquid.
The contact angle xcex8 may be computed using Young""s equation which may be expressed as:
xcex3lv cos xcex8=xcex3svxe2x88x92xcex3sl
in which xcex3sv and xcex3sl are the solid-vapor and solid-liquid surface tensions, respectively. For a ceramic/metal melt system, the difference between the solid-vapor surface tension xcex3sv and the sold-vacuum surface tension xcex3so is negligible. Through simple thermodynamic calculations, the following relation for the contact angle xcex8 is reached:
cos xcex8=(2Vsl/Vll)xe2x88x921
in which Vsl and Vll are the solid-liquid and liquid-liquid interaction energies, respectively.
In metallic liquids the liquid-liquid interaction energy Vll is relatively strong. Thus, when injecting metallic liquids in prior art techniques the contact angle xcex8 was assumed to be close to 180xc2x0.
With such an assumption, the external pressure needed to drive the molten metal into a channel with diameter D is Paxe2x89xa7xe2x88x924xcex3lv/D.
As an example, the solid-liquid surface tension xcex3sl of liquid bismuth is about 375 dyn/cm. Assuming the contact angle xcex8 is 180xc2x0, a pressure of 1,150 bar is needed to fill a pore having a diameter of about 13 nm. Such a high pressure can be achieved by a hydrostatic press. The melting temperature of bismuth, however, is 271.5xc2x0 C. Thus, to fill a pore having a diameter of about 13 nm, a hydrostatic press must provide a pressure of 1,150 bar at a temperature of at least 271.5xc2x0 C. Due to the combination of high pressure and high temperature, and the corresponding problems associated with operating hydrostatic equipment at such high temperatures and pressures, it was heretofore not practically possible to inject liquids and, in particular, liquid metals into pores having relatively small diameters.
Moreover, even if relatively small diameter pores in anodic alumina could be filled, as explained above, the anodic alumina itself is typically unsuitable as a template for quantum wires, due to the lack of a systematic technique for controlling the diameter, a length and packing density of the pores in the anodic alumina.
It would, therefore, be desirable to provide a technique for fabricating an array of nanowires having a relatively small diameter, a relatively close spacing and a relatively long length. It would also be desirable to provide a technique for fabricating nanowires which does not depend upon the selection of chemical precursors. It would also be desirable to provide a technique which can be used to fabricate continuous wires having a relatively long length and which does not require high pressure injection of molten materials at relatively high temperatures. It would also be desirable to provide a template having pores therein with pore diameters which are relatively uniform. It would also be desirable to provide a technique for filling substrate pores having relatively small diameters. It would also be desirable to provide a technique for systematically controlling the pore diameter, pore length and center-to-center spacing of pores in an anodic aluminum oxide template.
In accordance with the present invention, an array of nanowires includes a substrate having a plurality of non-interconnected pores each of the pores having pore diameter which does not vary by more than one hundred percent and a material continuously filled in each of the plurality of pores of the substrate wherein the material has characteristic such that the material can become a quantum wire having a length not less than three microns. With this particular arrangement, an array of non-interconnected nanowires which can be used in a semiconductor device, an optical device or a thermoelectric device is provided. The substrate may, for example, be provided from a metal such as aluminum or an aluminum alloy in sheet or metal form having a surface layer of aluminum oxide thereon. Alternatively, the substrate may be provided from a mesoporous material such as a material from the silicate/aluminosilicate mesoporous molecular sieves. The material disposed in the pores may be provided as bismuth (Bi), or any other material capable of becoming a quantum wire. The substrate pores are provided having a wall composition or a surface property which reduces the contact angle between the material continuously filling the pores and the pore wall. Thus, the substrate pores can be filled utilizing relatively little, if any, pressure.
In accordance with a further aspect of the present invention, a method for providing a substrate having pores with walls having reduced contact angles includes the step of treating the pore wall with an acid solution to change at least one of a pore wall composition and a pore wall surface property. With this particular arrangement, a substrate having pores which can be filled without high pressure injection of a molten material at a relatively high temperature is provided. In one embodiment, the substrate is provided as an anodic aluminum oxide film, which is prepared by the anodic oxidation of aluminum in an acidic electrolyte. The electrolyte solution is selected to provide an anodic aluminum oxide film having a pore with a wall surface composition which is different than aluminum oxide. Thus, the pore wall composition or properties are modified during an anodization process. The modified pore walls result in a contact angle between the pore wall and a filling material which allows the pore to be continuously filled with a material without the use of high pressure injection techniques. Alternatively, the pore wall composition or surface properties may be modified after the anodizing process by applying a solution of H2SO4 to the pore wall to thus change the composition or surface properties of the pore walls to provide the pore walls having a contact angle which allows molten material to fill the pores without the use of high pressure injection techniques. Alternatively still, the composition or surface properties of the pore walls may be modified by depositing a desired surface species on the pore wall by a vapor deposition technique, for example. In another embodiment, the substrate is provide as mesoporous MCM-41. The pores in the mesoporous material may also be treated such that the contact angle between the pore walls and the material filling the pores allows the material to be drawn into the pores with little or no pressure.
In accordance with a still further aspect of the present invention a technique for fabricating nanowires includes the steps of treating the pores of an anodic aluminum oxide film to improve a contact angle of a pore wall, melting metal under vacuum and injecting the molten metal under pressure into the pores of the anodic aluminum oxide film to produce continuous nanowires. With this particular arrangement, a dense array of continuous nanowires having relatively small diameters which can be utilized in transport-related applications is provided. In one embodiment, the anodic aluminum oxide film has a plurality of pores formed therein and the technique is used to provide a dense array of nanowires. Thus, the process of the present invention can be utilized to generate large areas of highly regular and densely-packed nanowire arrays. Moreover, the process does not require clean room technology as is necessary for fabrication of quantum well superlattice systems. Therefore, a relatively simple and inexpensive technique for fabrication of densely-packed arrays of continuous nanowires is provided. Another advantage of this technique is that it can be applied to a wide range of materials including low melting temperature metals, alloys, semiconductors, and organic polymer gels and thus the technique is versatile. In one particular application an array of bismuth nanowires having average wire diameters as small as 13 nm, lengths of 30-50 xcexcm, and a packing density greater than 4.6xc3x971010 cmxe2x88x922 is provided.
In accordance with still a further aspect of the present invention a method to systematically change the channel diameter and channel packing density of anodic aluminum oxide film includes the steps of anodizing an aluminum substrate with a particular one of a plurality of electrolytes at a predetermined voltage level, a predetermined temperature and a predetermined current and exposing pores in the anodized aluminum substrate to an acid which modifies either the composition or a surface property of a surface of the pore walls. With this particular arrangement, a systematic method for providing aluminum oxide film having particular characteristics is provided. The method can be used to provide, for example, an anodic aluminum oxide film having a particular pore diameter in the range of pore diameters extending from about 8 nm to about 200 nm. Moreover, with this technique, the pore diameters do not vary by more than one-hundred percent. Thus, with this technique, an anodic aluminum oxide film having an average pore diameter of 8 nm and having a desired channel length and structural regularity can be provided.
The solid-liquid surface tension xcex3sl, depends on the surface properties of the solid in which the pores are formed and is not always small compared to the liquid-liquid surface tension xcex3ll. By changing the composition or a surface property of the pore wall, a desired wall surface for individual liquids can be produced, thereby reducing the contact angle xcex8. It has been recognized that in the pressure injection process, the contact angle xcex8 plays an important role. Specifically, if the contact angle xcex8 is less than 90xc2x0, the capillary pressure 4xcex3lv cos xcex8/D is positive, so that this pressure itself is able to drive the liquid into the pores even when the pores are provided having a relatively small diameter. If the contact angle xcex8 is greater than 90xc2x0, however, an external pressure higher than xe2x88x924xcex3lv cos xcex8/D is needed. For an anodic alumina template the portion of alumina that is closed to the internal surface of the channels is contaminated by anions from the anodizing electrolyte. This means that solid-liquid surface tension xcex3sl depends on both the specific metal melt and the electrolyte type. Thus by appropriately selecting a suitable electrolyte the contact angle can be reduced and molten materials may be driven into pores having diameters at least as low as 8 nm with relatively low or no pressure. In one experiment, an anodic alumina template was prepared using a sulfuric acid solution and molten bismuth was successfully driven into pores with diameters as small as 13 nm at a pressure of 315 bar. The sulfuric acid electrolyte used for bismuth may also be appropriate for other molten metals. It is recognized, however, that the use of an anodizing electrolyte or a pore etching solution to adjust the pore wall composition or surface property may not be applicable to every molten material or even to every molten metal. If a suitable acid solution cannot be found for injecting a particular molten material of interest, a vapor deposition of a desired surface species before the pressure injection process may be performed in place of or in addition to, the use of an anodizing electrolyte or a pore etching solution to thus control the composition or surface properties of the pore wall to provide a pore wall surface which favors the interaction between the pore wall surface and molten material of interest. This technique allows the use of a modest pressure to drive a molten or liquid material into the pores and thus existing manufacturing and processing equipment can be used.
In accordance with a still further aspect of the present invention, a method to control the channel diameter of an anodic alumina film and the ratio of a pore diameter to the cell size during and after an anodizing process is described. With this particular technique, a method for controlling the pore structure of an anodic alumina film is produced. A vacuum melting and pressure injection process can be used to then fill an array of densely packed pores to thus generate continuous and dense nanowires useful in many electronic applications. Due to the high thermal and chemical stability of the anodic aluminum oxide film, the pressure injection process can be applied to other low melting temperature metals, semiconductors, alloys, and polymer gels. In one particular experiment an array of bismuth nanowires having an average diameter of about 13 nm, a length of about 30 xcexcm and a 7.1xc3x971010 cm-xe2x88x922 packing density was fabricated. Moreover, the individual wires were provided having a single crystal lattice structure.
The nanowire array composites fabricated in accordance with the techniques of the present invention find applicability in a wide range of fields including but not limited to use in electronic devices, photonics, high Tc superconductivity, thermoelectricity, chemical gas sensors and chemical gas separation. In particular, the 1D quantum wire systems find application in a wide range of technical fields of use.